The mechanisms involved in establishing the segmentation pattern in Drosophila embryos will be investigated. This is an excellent model system for understanding at the molecular level the formation of body pattern in multicellular organisms. These studies should thus provide basic knowledge relevant to the understanding of early development and the nature of birth defects. This proposal focusses on runt, a gene required early in Drosophila development for normal segmentation. Molecularly cloned runt gene DNA will be used to generate probes for both the RNA transcripts from the gene and the runt protein. These will be used to assay the spatial and temporal regulation of runt expression during embryogenesis. Several genes which are known to be required for normal segmentation will be tested for their effects on runt expression. Mutagenesis screens will also be done to find as of yet unidentified genes which are involved in regulating runt activity. These studies on trans-acting factors should help to define a genetic framework for the process of segmentation and provide preliminary indications of the relative roles of the different genes. This research will be complemented by an investigation into the DNA sequences required in cis for appropriate expression of runt. Alterations in the gene will be generated in vitro and then reintroduced by germ-line transformation, or alternatively produced by further in vivo mutagenesis. The goal of these studies is to identify potential sites of interaction with trans-acting factors. This work should provide a foundation for future research on the molecular mechanisms involved in segmental pattern formation.